Globally, the average rise in life expectancy between 2016 and 2040 is estimated to be 4.4 years, though the US will be far below.

Americans' average life expectancy will go up only 1.1 years to 79.8, and the US will drop from 43rd to 64th place in world rankings.

China, which had a life expectancy of 76.3 years in 2016, is predicted to hit an average of 81.9 years by 2040. The increase would put China in 39th place.

Four countries in sub-Saharan Africa will have life expectancies of less than 65 years by 2040, the study found. Of the 195 countries and territories analyzed, Lesotho will have the lowest life expectancy, with an average of 57.3 years.

In a worst-case result, Lesotho's life expectancy could be as low as 45.3 years, according to the study. The bottom spot is currently occupied by the Central African Republic, whose life expectancy the CIA puts at 52.8 years.

The study's authors, who work at the University of Washington's Institute for Health Metrics and Evaluation, relied on existing data about diseases to make their predictions. For example, they incorporated information about rates of cancer, diabetes, and HIV/AIDS.

Alcohol use, tobacco use, high blood pressure, high body-mass index, and high blood sugar cause the highest number of premature deaths, the study said, and these moralities are expected to increase. Though other risks, such as unsafe water and child malnutrition, are expected to decrease, the overall life expectancy will go up more slowly than before.